1. Field of the Invention
This invention relates to a process for preparing 1-tetrahydronaphthalone which comprises oxidizing tetrahydronaphthalene by a liquid phase oxidation with molecular oxygen or a molecular oxygen-containing gas. More particularly, this invention relates to an improved process for preparing 1-tetrahydronaphthalone in a high selectivity from tetrahydronaphthalene which comprises oxidizing tetrahydronaphthalene by a liquid phase oxidation with molecular oxygen or a molecular oxygen-contianing gas in the presence of a catalyst system comprising a chromium salt of naphthenic acid and an alkyl-substituted pyridine compound having the formula ##SPC1## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5, which can be the same or different, each represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms and at least one of R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.5 is an alkyl group, thereby minimizing the formation of 1-tetrahydronaphthol as a by-product.
2. Description of the Prior Art
It is well known that 1-tetrahydronaphthalone can be converted by dehydrogenation into .alpha.-naphthol which is an industrially important raw material having a wide variety of utilities, e.g., as an intermediate of dyes, a raw material of agricultural agents, etc.
Hitherto, various processes for producing 1-tetrahydronaphthalone from tetrahydronaphthalene have been studied and known to the art, for example, (1) a process comprising air-oxidation of tetrahydronaphthalene in the presence of a cobalt salt or an iron salt, (2) a process comprising air-oxidation of tetrahydronaphthalene using a catalyst system of a chromium acetate-2-methyl-5-ethylpyridine complex as disclosed in U.S. Pat. No. 3,404,185, and (3) a process comprising oxidation of tetrahydronaphthalene in the presence of a catalyst system comprising a cobalt naphthenate having incorporated therein an amine, a pyridine compound and the like.
In the prior art process (1) above, the selectivity to 1-tetrahydronaphthalone is low thereby producing 1-tetrahydronaphthaol as a by-product in a large proportion. The resulting 1-tetrahydronaphthol has little utility in industry and, in addition, causes various undesirable problems, for example, clogging of reactors and conduits in a purification step of the resulting product such as distillation since 1-tetrahydronaphthol tends to be converted further into secondary by-products through dehydration, dimerization or the like.
Accordingly, the formation of a large amount of 1-tetrahydronaphthol in the production of 1-tetrahydronaphthalone is one of the serious problems. Thus, it is very important in the liquid phase oxidation of tetrahydronaphthalene to minimize as low as possible the formation of 1-tetrahydronaphthol as a by-product and to produce 1-tetrahydronaphthalone in high selectivity, i.e., to increase a molar ratio of 1-tetrahydronaphthalone/1-tetrahydronaphthol in the reaction mixture obtained by the liquid phase oxidation of tetrahydronaphthalene.
In the trial of the prior art process (2) above by the present inventors, this process revealed a 1-tetrahydronaphthalone/1-tetrahydronaphthol molar ratio of 19.4 under conditions which would provide the optimum result in a batch process, as shown hereinafter in greater detail in Comparative Examples, but the process revealed a markedly low molar ratio of 1-tetrahydronaphthalone/1-tetrahydronaphthol, i.e., 3,9, in a continuous process which is considered to be a practically important process in the production of 1-tetrahydronaphthalone on an industrial scale. Further, this prior art process was found to have a serious disadvantage as an industrial process in that, since chromium acetate used in this process does not dissolve in tetrahydronaphthalene, a solid precipitate is formed during the oxidation reaction and deposited in the reactor and the conduits thereby making it difficult to operate the process for a long period of time.